The present invention relates to a system for controlling the air-fuel ratio of mixture for an automotive engine, and more particularly to a learning control system for updating data stored in a table so as to properly drive a motor vehicle.
In a learning control system, updating of data is performed with new data obtained during the steady state of engine operation. Accordingly, means for determining whether the engine operation is in the steady state is necessary. A conventional learning control system (for example U.S. Pat. No. 4,309,971) has a matrix (two-dimensional lattice) comprising a plurality of divisions, each representing engine operating variables such as engine speed and engine load. When the variables continue for a predetermined period of time in one of the divisions, it is determined that the engine is in the steady state. On the other hand, a three-dimensional look-up table is provided in which a matrix coincides with the matrix for determining the steady state. Data in the look-up table is updated with new data obtained during steady state.
In such an air-fuel ratio control system, if an O.sub.2 -sensor for obtaining information for updating data deteriorates, the difference between a maximum value and a minimum value of output voltage of the O.sub.2 -sensor decreases. When the difference in output voltage decreases, the system does not perform its function in providing proper data. Accordingly, the driveability of a vehicle decreases and fuel consumption increases.